1. Field of the Invention
The present invention relates to a polycarbodiimide solution and methods for producing the same, and particularly, to a high-molecular weight polycarbodiimide solution with excellent stability in preservation which can be obtained by appropriately choosing reaction conditions such as a solvent used in the production of the solution, and to a method for producing said polycarbodiimide solution.
2. Prior Art
It is known that polycarbodiimides, particularly aromatic polycarbodiimides, have a high level of heat resistance, and such polycarbodiimides are therefore used as thermosetting resin materials for molding which are hot-pressed in a powder form.
A method for producing said polycarbodiimides is disclosed in Japanese Patent Laid-Open No. 51-61599, and other methods are reported in D. J. Lyman et al., Die Makromol. Chem., 67, 1 (1963) and E. Dyer et al., J. Amer. Chem. Soc., 80, 5495 (1958) . Each of these methods uses, as a polymerization solvent, a hydrocarbon such as benzene, xylene, decalin, toluene or the like, or, o-dichlorobenzene, chlorobenzene, cyclohexanone, a mixture of dimethyl sulfoxide and chlorobenzene (1:1), N-methylpyrrolidone or the like, and uses as a monomer, one of various organic diisocyanates such as 4,4'-diphenylmethane diisocyanate, tolylene diisocyanate and the like or mixtures thereof, in a concentration within the range of 10 to 25% by weight.
The polycarbodiimides obtained by the production methods mentioned in the above documents are powdery, and the films also mentioned in the above documents are produced by extracting the polycarbodiimides from the reaction system after 10 minutes from the start of reaction and then casting it.
And moreover, since gelation or precipitation occurs in the reaction system over 10 minutes after the start of the above reaction, the polycarbodiimides thus obtained cannot be used in the form of a stable solution. Namely, high-molecular weight polycarbodiimide solutions having excellent stability cannot be obtained by the above methods.
Another method for producing polycarbodiimides is a method in which the terminal isocyanate groups are sealed. L. M. Alberino et al., J. Appl. Polym. Sci., 21, 1999 (1977) and T. W. Campbell, J. Org. Chem., 28, 2069 (1963) also show methods for producing polycarbodiimides from organic diisocyanates in which the molecular weight of the polycarbodiimides produced is controlled by adding the organic monoisocyanate to the reaction system. In any one of these production methods, reaction is effected by using as a solvent a hydrocarbon such as benzene, xylene, petroleum ether or the like to produce a polycarbodiimide as a precipitate product. Namely, polycarbodiimide solutions cannot be obtained by the above mentioned production methods, and it is obvious that polycarbodiimides having a high molecular weight cannot be obtained.